templates: Offically make templates extensible

This supercedes #4757 (and #4568) by making template extensions
configurable.

The previous implementation was never documented AFAIK and had only
1 consumer, which I'll notify as a courtesy.

modules/caddyhttp/templates/caddyfile.go

@@ -15,6 +15,9 @@
 package templates
 
 import (
+	"github.com/caddyserver/caddy/v2"
+	"github.com/caddyserver/caddy/v2/caddyconfig"
+	"github.com/caddyserver/caddy/v2/caddyconfig/caddyfile"
 	"github.com/caddyserver/caddy/v2/caddyconfig/httpcaddyfile"
 	"github.com/caddyserver/caddy/v2/modules/caddyhttp"
 )
@@ -49,6 +52,29 @@ func parseCaddyfile(h httpcaddyfile.Helper) (caddyhttp.MiddlewareHandler, error)
 				if !h.Args(&t.FileRoot) {
 					return nil, h.ArgErr()
 				}
+			case "extensions":
+				if h.NextArg() {
+					return nil, h.ArgErr()
+				}
+				if t.ExtensionsRaw != nil {
+					return nil, h.Err("extensions already specified")
+				}
+				for nesting := h.Nesting(); h.NextBlock(nesting); {
+					extensionModuleName := h.Val()
+					modID := "http.handlers.templates.functions." + extensionModuleName
+					unm, err := caddyfile.UnmarshalModule(h.Dispenser, modID)
+					if err != nil {
+						return nil, err
+					}
+					cf, ok := unm.(CustomFunctions)
+					if !ok {
+						return nil, h.Errf("module %s (%T) does not provide template functions", modID, unm)
+					}
+					if t.ExtensionsRaw == nil {
+						t.ExtensionsRaw = make(caddy.ModuleMap)
+					}
+					t.ExtensionsRaw[extensionModuleName] = caddyconfig.JSON(cf, nil)
+				}
 			}
 		}
 	}

modules/caddyhttp/templates/templates.go

@@ -319,6 +319,10 @@ type Templates struct {
 	// the opening and closing delimiters. Default: `["{{", "}}"]`
 	Delimiters []string `json:"delimiters,omitempty"`
 
+	// Extensions adds functions to the template's func map. These often
+	// act as components on web pages, for example.
+	ExtensionsRaw caddy.ModuleMap `json:"match,omitempty" caddy:"namespace=http.handlers.templates.functions"`
+
 	customFuncs []template.FuncMap
 }
 
@@ -338,17 +342,13 @@ func (Templates) CaddyModule() caddy.ModuleInfo {
 
 // Provision provisions t.
 func (t *Templates) Provision(ctx caddy.Context) error {
-	fnModInfos := caddy.GetModules("http.handlers.templates.functions")
-	customFuncs := make([]template.FuncMap, 0, len(fnModInfos))
-	for _, modInfo := range fnModInfos {
-		mod := modInfo.New()
-		fnMod, ok := mod.(CustomFunctions)
-		if !ok {
-			return fmt.Errorf("module %q does not satisfy the CustomFunctions interface", modInfo.ID)
-		}
-		customFuncs = append(customFuncs, fnMod.CustomTemplateFunctions())
+	mods, err := ctx.LoadModule(t, "ExtensionsRaw")
+	if err != nil {
+		return fmt.Errorf("loading template extensions: %v", err)
+	}
+	for _, modIface := range mods.(map[string]any) {
+		t.customFuncs = append(t.customFuncs, modIface.(CustomFunctions).CustomTemplateFunctions())
 	}
-	t.customFuncs = customFuncs
 
 	if t.MIMETypes == nil {
 		t.MIMETypes = defaultMIMETypes